Break in capabilities.php if $post_type is null

[nofearinc]

There is an edge case in capabilities.php where $post_type could be null while verifying capabilities as there is some database data assigned to a post_type that is no longer registered.

Steps to reproduce on a clean install:

1) Register a post type
2) Create an entry from this post type
3) Publish a comment for the post type
4) Remove the post type registering function (i.e. post type no longer exists)
5) Go to the Edit Comments listing in Admin panel

This is the sort of notices I get for comments related to the post type (all normal comments in registered post types are fine): ​https://gist.github.com/mpeshev/ccb317a7c2ceb5bbd385

Adding a sample patch addressing the issue - the global $wp_post_types has no information about the old post type, therefore get_post_type_object returns null. There might be a smarter way to address that - such as just returning the post type name, but I think it's safer.

Note: There are several other calls in the same file to the same function, so it probably has to be addressed there as well, but I was unable to trigger another related error.

[ocean90]

Duplicate of #16956.